The invention relates to a hand-guided electric tool having a motor and a pulse width modulator for generating a pulse width modulated signal for operating the motor. A unit is provided for reducing the EMC interferences emitted by the electric tool.
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1. A hand-held power tool comprising:
a motor;
a pulse width modulator for producing a pulse width modulated signal for operating the motor;
a device for modulating a carrier frequency of the pulse width modulated signal with a noise signal or a pseudorandom signal, thereby reducing EMC interference emitted by the power tool; and
a clock-pulse generator for producing a clock-pulse signal, the pulse width modulated signal being derived from the clock-pulse signal;
wherein a counter register is provided for counting the elapsed clock cycles of the clock signal, a fold-back value register and an overflow value register are provided, and the pulse width modulator
emits a first signal level if the value of the counter register is less than a fold-back value,
emits a second signal level if the value of the counter register is equal to or greater than the fold-back value,
sets the counter to a starting value if the value of the counter is equal to an overflow value,
sets the fold-back value and overflow value to random new values if the value of the counter is equal to a previous overflow value, with the ratio of the new fold-back value to the new overflow value being essentially equal to the ratio of a previous fold-back value to the previous overflow value.
2. The power tool as recited in
3. The power tool as recited in
4. The power tool as recited in
5. The power tool as recited in
6. The power tool as recited in
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This application is a 35 USC 371 application of PCT/EP2008/063855 filed on Oct. 15, 2008.
1. Field of the Invention
The present invention relates to a hand-held power tool.
2. Description of the Prior Art
Hand-held power tools such as drills, cordless screwdrivers, jigsaws, angle grinders, or electric hedge trimmers that are powered by batteries, rechargeable batteries, or a power cord are generally known. Power tools of this kind have electric motors, which, according to the prior art, are operated with a pulse width modulated voltage or, in the case of corded tools, by means of phase-angle control. As schematically depicted in
One problem when using the pulse width modulation method is the production and emission of harmonics. Because of the periodic switching back and forth between a high and low voltage level and the resulting current changes, harmonics are produced whose frequency is an uneven multiple of the modulation frequency fPWM of the pulse width modulation (
The object of the present invention is to disclose a device that reduces the amplitudes of the EMC interference emitted by a hand-held power tool.
The object underlying the invention is attained by means of a power tool with the defining characteristics according to the invention.
In one embodiment of the invention, a hand-held power tool has a motor and a pulse width modulator for producing a pulse width modulated signal for operating the motor. According to the invention, the emitted EMC interference is reduced by using filter elements such as capacitors, chokes, and combinations thereof.
In another embodiment, the emitted interference is reduced by flattening or smoothing the edges of the pulse width modulated signal, which reduces the share of high-frequency signal components, i.e. harmonics.
In another embodiment of the invention, the carrier frequency of the pulse width modulated signal can be modulated using a noise signal or pseudorandom signal.
The pulse width modulated signal with a random-modulated carrier frequency advantageously has no discrete spectral lines with multiples of the carrier frequency of the pulse width modulation. Instead, each of these spectral lines is spread out over a frequency band. This distributes the total power of each high-frequency signal component over a frequency interval and as a result, the amplitudes of the individual signal maxima decrease.
In another embodiment, a clock pulse produced by a clock-pulse generator is modulated by a clock-pulse modulator using a noise signal or pseudorandom signal; the modulated clock signal is supplied to a pulse width modulator, which produces a pulse width modulated signal with a noise-modulated carrier frequency. The noise signal or pseudorandom signal can be produced by an analog noise generator. The noise signal or pseudorandom signal can also be produced as a digital pseudorandom number and converted into an analog pseudorandom signal by a smoothing element.
In a preferred embodiment of the invention, a microcontroller is provided as the pulse width modulator.
In another preferred embodiment, the modulation of the carrier frequency of the pulse width modulated signal is carried out digitally by a microcontroller using a noise signal or pseudorandom signal.
In this embodiment of the invention, the noise modulation of the carrier frequency of the pulse width modulated signal can be advantageously implemented entirely at the software level. As a result, no additional hardware components are required, incurring no increase in costs or in the assembly complexity required to manufacture the power tool.
In the drawings, parts that are the same or function in the same manner have been provided with the same reference numerals. The invention is explained in greater detail below in conjunction with the accompanying drawings, in which:
Because of the fixed carrier frequency fPWM, the spectrum of the pulse width modulated voltage signal in
One possibility for reducing EMC interference is to use filter elements such as capacitors, chokes, and combinations thereof. The use of additional components, however, increases the size of the power tool 100 and the assembly complexity required for its manufacture, thus incurring additional costs.
Another possibility for reducing the interference emitted by the pulse width modulation is to flatten or smooth the edges of the pulse width modulated signal. This reduces the share of high-frequency signal components, i.e. harmonics. A flattening of the signal edges can, for example, be achieved by the fact that the driver module that produces the pulse width modulated signal is intentionally embodied to be slower, for example through the insertion of a series resistance. However, doing so also protracts the switching time of a power switch situated after the driver module, thereby increasing switching losses. The power consumption of a power tool 100 increases as a result. This can negatively affect the running time of a power tool 100 powered by a battery or rechargeable battery.
Another possibility for reducing the emitted EMC interference is to reduce the amplitudes of the harmonics of the pulse width modulated signal by using a noise signal or pseudorandom signal to modulate the carrier frequency of the pulse width modulated signal. This is schematically depicted in
The pulse width modulator 170 has a counter 171, a defined fold-back value 172, and a defined overflow value 173. The counter 171, the fold-back value 172, and the overflow value 173 can, for example, be embodied in the form of a memory register of the microcontroller. The pulse width modulator 170 increases the value of the counter 171 by the number 1 with each clock cycle of the constant clock pulse 111. If the value of the counter 171 is less than the fold-back value 172, then the pulse width modulator 170 emits a high voltage level as a set point voltage (a noise-modulated carrier frequency 106). If the value of the counter 171 is greater than or equal to the fold-back value 172, then the pulse width modulator 170 emits a low voltage level as a set point voltage (a noise-modulated carrier frequency 106). If the value of the counter 171 is less than the overflow value 173, then the pulse width modulator 170 waits for the next clock cycle of the constant clock pulse 111 in order to then repeat the above-described process, starting from the increase of the counter 171.
If the value of the counter 171 is equal to the overflow value 173, then the value of the counter 171 is reset to a starting value, for example the value 0. Otherwise, the fold-back value 172 and the overflow value 173 for the subsequent clock cycle of the pulse width modulated set point voltage are modulated with a noise-modulated carrier frequency 106. The overflow value 173 determines the period length of the carrier frequency of the pulse width modulated set point voltage with a noise-modulated carrier frequency 106. The ratio of the fold-back value 172 to the overflow value 173 yields the mark/space ratio of the pulse width modulated set point voltage with a noise-modulated carrier frequency 106 and should vary as little as possible between the individual clock cycles of the carrier frequency of the pulse width modulated set point voltage with a noise-modulated carrier frequency 106. Ideally, the fold-back value 172 and the overflow value 173 are therefore multiplied by the digital pseudorandom number 161. In an alternative embodiment of the power tool 100 according to the invention, the digital random number 161 is added to the fold-back value 172 and the overflow value 173. This embodiment has the advantage that it is less computationally demanding for the pulse width modulator 170 to execute an addition than to execute a multiplication. If the value of the digital random number 161 is small compared to the fold-back value 172, then the resulting variation of the mark/space ratio of the pulse width modulated set point voltage with a noise-modulated carrier frequency 106 is negligibly low and averages out over the course of time. In a particularly preferred embodiment, the pulse width modulator 170 has an additional register for storing a constant fold-back value and an additional register for storing a constant overflow value. The new fold-back value 172 and the new overflow value 173 are calculated in each new period of the carrier frequency of the pulse width modulated set point voltage with a noise-modulated carrier frequency 106, based on the constant fold-back value and overflow value stored in the additional registers. This prevents the overflow value 173 and the fold-back value 172 from deviating too far from their initial values over time.
A voltage source 101 integrated into the power tool 100 emits a constant voltage 102. A power switch 103 uses the constant voltage 102 and the pulse width modulated set point voltage with a noise-modulated carrier frequency 106 to produce a pulse width modulated voltage with a noise-modulated carrier frequency 104, which drives a motor 105 of the power tool 100.
The foregoing relates to the preferred exemplary embodiments of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.
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Mar 29 2010 | BOSCH, VOLKER | Robert Bosch GmbH | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 024702 | /0366 |
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